The present invention relates to an optical transmitter of an external modulation method employed in an optical communication system and a control method of a bias voltage to an optical modulator employed therein. More particularly, this invention relates to an optical transmitter using an optical modulator of Mach-Zehnder type and a control method of a bias voltage to an optical modulator employed therein.
Conventionally, the optical communication system uses the direct modulation method. In this method, light intensity signal proportional to an electric signal serving as a driving current is obtained by generating an optical modulation signal with a driving current for a laser diode. However, in an ultra-fast broadband optical communication system having a transmission rate exceeding several Gbit/s, the wavelength of light changes at the direct modulation, which is known as chirping and limits a transmission capacity.
On the other hand, the chirping occurs less frequently in the external modulation method. Furthermore, in the external modulation method, modulation is relatively easy in an operating band of 10 GHz or higher, and therefore, has been applied to an ultra-fast broadband optical communication system with a large capacity. The most popular optical modulator as the external modulator is a Mach-Zehnder optical modulator using lithium niobate (LiNbO3).
An output optical signal I(t) modulated by a modulation signal S(t) by using the Mach-Zehnder optical modulator is expressed by Equation (1):
I(t)=k{1+cos (xcex2xc2x7S(t)+xcex4)}xe2x80x83xe2x80x83(1)
where k represents a proportion coefficient, xcex2 represents a degree of modulation, and xcex4 represents a phase at the operating point of the Mach-Zehnder optical modulator.
Given that the modulation signal S(t) is a binary digital signal, the degree of modulation xcex2 is xcex2=xcfx80, and the initial phase xcex4 is xcex4=xcfx80/2 by applying an adequate DC voltage (bias voltage) to the Mach-Zehnder optical modulator, then the Mach-Zehnder optical modulator outputs the output optical signal I(t) that switches ON/OFF in proportionate to the modulation signal S(t).
Given that the degree of modulation xcex2 is xcex2=2xcfx80, the initial phase xcex4 is xcex4=0 by applying an adequate bias voltage to the Mach-Zehnder optical modulator, and the modulation signal S(t) is used, then when a sine wave having a repeating frequency R is input, the output optical signal I(t) is expressed by equation (2).
I(t)=k{1+cos (2xcfx80xc2x7sin (2xcfx80R(t)))xe2x80x83xe2x80x83(2)
Hence, the output optical signal I (t) expressed by equation (2) is output as an optical signal that switches ON/OFF at a repeating frequency 2R that is double the repeating frequency R.
There would be no problem if the phase xcex4 is constant. However, a typical optical modulator using lithium niobate has a problem that the operating point undesirably drifts. Two types of drift are known. That is, a thermal drift induced by the pyroelectric effect caused by a temperature change; and a DC drift induced by a charge distribution over the surface of the element of the optical modulator produced by the bias voltage applied to the electrode of the optical modulator. In order to compensate variance of the operating point caused by these types of drift, it is necessary to apply a bias voltage to the optical modulator in such a manner so as to attain an optimal operating point.
FIG. 8 is a block diagram depicting an arrangement of a conventional optical transmitter capable of stabilizing a bias voltage applied to the optical modulator using lithium niobate (see Japanese Patent Application Laid-Open No. 5-142504). Continuous optical signals emitted from a light source 101 are input into a Mach-Zehnder optical modulator 103 using lithium niobate. A terminator 114 is connected to the Mach-Zehnder optical modulator 103 and a driving signal for driving the Mach-Zehnder optical modulator 103 and a bias voltage are applied to the Mach-Zehnder optical modulator 103 through a node TT.
Output optical signal modulated by the Mach-Zehnder optical modulator 103 is output to an output terminal 120 through a branching filter 104, and a part of the output optical signal is input into a photo diode 105. The photo diode 105 converts the input part of the output optical signal into an electric signal, amplifies the electric signal by means of a pre-amplifier 106, and outputs the same to a synchronous detector circuit 107.
The synchronous detector circuit 107 conducts synchronous detection between the electric signal input from the pre-amplifier 106 and a low frequency signal output from a dither signal generator 112. The synchronous detector circuit 107 includes a mixer 117, which mixes the electric signal input from the pre-amplifier 106 with the low frequency signal output from the dither signal generator 112. The resulting mixed signal is input into a low pass filter 109 through an operational amplifier 108, and the signal having passed through the low pass filter 109 is output to a bias voltage control circuit 110.
The bias voltage control circuit 110 includes a DC voltage 118 and an adder 119. The adder 119 adds an output signal from the synchronous detector circuit 107 and a bias voltage output from a DC power source 118, and outputs the sum as a bias voltage to the Mach-Zehnder optical modulator 103 from the node TT through an inductor 111. On the other hand, a driving signal is input into an input terminal 121 and output to a low frequency superimposing circuit 113 through a driving circuit 124. The low frequency superimposing circuit 113 superimposes the input driving signal and a low frequency signal output from the dither signal generator 112, and applies the resulting signal as a driving signal to the Mach-Zehnder optical modulator 103 from the node TT through a capacitor. Hence, both the driving signal superimposed with the low frequency signal and the bias voltage under the bias voltage control are applied to the Mach-Zehnder optical modulator 103 from the node TT.
How the bias voltage to the Mach-Zehnder optical modulator is controlled in the conventional optical transmitter will now be explained with reference to FIG. 9 to FIG. 11. FIG. 9 is a view explaining a modulation operation of the Mach-Zehnder optical modulator 103 when a bias voltage (phase xcex4) is at an adequate value. Operating characteristic curve 130 of the Mach-Zehnder optical modulator 103 represents the operating characteristic curve expressed by the equation (1), and indicates a state where the bias voltage (phase xcex4) is adequately set. In this case, upon input into the Mach-Zehnder optical modulator 103, a driving signal (input signal) 131, which has been superimposed with the low frequency signal, is modulated by the operating characteristic curve 130 and output as an output optical signal 132. The output optical signal 132 does not include a low frequency component (f[Hz]) of the low frequency signal superimposed on the driving signal, and a low frequency component (2f[Hz]) double the low frequency component (f[Hz]) is generated. Thus, after a part of the output optical signal 132 is received by the photo diode 105, amplified by the pre-amplifier 106, and let undergo the synchronous detection by the synchronous detector circuit 107, the resulting signal outputs xe2x80x9c0xe2x80x9d. In this case, because no signal component is added by the adder 119 of the bias voltage control circuit 110, the current bias voltage is maintained and applied intact to the Mach-Zehnder optical modulator 103.
On the other hand, FIG. 10 is a view explaining a modulation operation by the Mach-Zehnder optical modulator 103 when the bias voltage is at a relatively high value compared with an adequate value. Operating characteristic curve 140 of the Mach-Zehnder optical modulator 103 indicates a state where the bias voltage is set to a relatively high value compared with an adequate value. In this case, upon input into the Mach-Zehnder optical modulator 103, a driving signal 141, which is the same as the driving signal 131 superimposed with the low frequency signal, is modulated by the operating characteristic curve 140 and output as an output optical signal 142. The output optical signal 142 includes a low frequency component (f[Hz]) of the low frequency signal superimposed on the driving signal, and the phase of the low frequency component (f[Hz]) is inverted with respect to the phase of the low frequency component (f[Hz]) superimposed on the driving signal. Hence, the synchronous detector circuit 107 conducts synchronous detection of the low frequency component (f[Hz]),and outputs a xe2x80x9cnegativexe2x80x9d voltage to the bias voltage control circuit 110. In this case, the adder 119 of the bias voltage control circuit 110 adds the negative voltage to the bias voltage output from the DC power source 118, thereby effecting control to reduce the current bias voltage so as to be approximated to an adequate bias voltage.
Similarly, FIG. 11 is a view explaining a modulation operation by the Mach-Zehnder optical modulator 103 when the bias voltage is at a relatively low value compared with an adequate value. Operating characteristic curve 150 of the Mach-Zehnder optical modulator 103 indicates a state where the bias voltage is set at a relatively low value compared with an adequate value. In this case, upon input into the Mach-Zehnder optical modulator 103, a driving signal 151, which is the same as the driving signal 131 superimposed with the low frequency signal, is modulated by the operating characteristic curve 150 and output as an output optical signal 152. The output optical signal 152 includes a low frequency component (f[Hz]) of the low frequency signal superimposed on the driving signal, and the phase of the low frequency component (f[Hz]) coincides with the phase of a low frequency component (f[Hz]) superimposed on the driving signal. Hence, the synchronous detector circuit 107 conducts synchronous detection of the low frequency component (f[Hz]) and outputs a xe2x80x9cpositivexe2x80x9d voltage to the bias voltage control circuit 110. In this case, the adder 119 of the bias voltage control circuit 110 adds the positive voltage to the bias voltage output from the DC power source 118, and effects control to increase the current bias voltage so as to be approximated to an adequate bias voltage.
Thus, in the control method of a bias voltage applied to the Mach-Zehnder optical modulator in the conventional optical transmitter, a part of the output optical signal output from the Mach-Zehnder optical modulator 103 is detected, and the synchronous detector circuit 107 generates an error signal corresponding to displacement of the bias voltage from the optimal operating point, whereby the bias voltage control circuit 110 controls the bias voltage to make the error signal smaller, thereby maintaining the bias voltage in a stable manner.
Incidentally, in the conventional method, the low frequency signal is superimposed on the driving signal. However, the low frequency superimposing circuit 113 for superimposing the low frequency signal on the driving signal uses devices, such as a voltage control attenuator and a voltage control variable gain amplifier, which are not shown in the diagrams. Therefore, when the band of a driving signal reaches 10 GHz or higher, the operating band for these devices become insufficient, and this causes waveform deformation of the driving signal to be applied to the Mach-Zehnder optical modulator 103, resulting in a problem that the quality of the output optical signal is deteriorated.
Furthermore, in the Mach-Zehnder optical modulator 103 of the conventional optical transmitter, the low frequency signal, that is a dither signal, has to be superimposed on the driving signal to monitor the drift of the optimal operating point at the modulation. This demands the dither signal generator 112 and low frequency superimposing circuit 113, thereby posing a problem that the device cannot be made more compact and lighter.
The conventional optical transmitter obtains an output optical signal in proportionate to the repeating frequency R of the driving signal. However, there has been also an increasing need to stably control the bias voltage to the Mach-Zehnder optical modulator employed in the optical transmitter which outputs an output optical signal having the repeating frequency 2R that is double the repeating frequency R of the driving signal.
It is an object of this invention to provide an optical transmitter capable of preventing deterioration of the quality of an output optical signal by readily effecting stabilization control of a bias voltage and readily effecting stabilization control of a bias voltage when outputting an output optical signal having a repeating frequency 2R that is double the repeating frequency R of the driving signal when the band of a driving signal reaches 10 GHz or higher. It is another object of this invention to provide a method of controlling a bias voltage to an optical modulator employed in an optical transmitter.
In the optical transmitter according to one aspect of this invention, a driving unit inputs the driving signal into the Mach-Zehnder optical modulator to modulate the optical signal input from the light source to be output. A converting unit takes out a part of the optical signal output from the optical modulator to be converted into an electric signal. An extracting unit extracts a frequency component of the driving signal contained in the converted electric signal obtained by the converting unit. A phase comparing unit compares phases of the driving signal input into the driving unit and the frequency component of the driving signal extracted by the extracting unit. A bias voltage control unit effects feedback control on a bias voltage to be applied to the optical modulator based on a result of the phase comparison.
In the control method of a bias voltage to an optical modulator employed in an optical transmitter according to another aspect of this invention, a part of an optical signal output from a Mach-Zehnder optical modulator is taken out to be converted into an electric signal. Then, a frequency component of the driving signal contained in the converted electric signal is extracted. Furthermore, phases of the input signal and the extracted frequency component of the driving signal are compared. Finally, a bias voltage to be applied to the optical modulator is fed back under control based on a result of phase comparison.
Other objects and features of this invention will become apparent from the following description with reference to the accompanying drawings.